In fiber laser sensors, changes in the refractive index of the fiber on account of external influences, such as e.g. pressure and temperature changes, are converted into a change in the laser wavelength or into a change in the beat frequency between two longitudinal laser modes.
A fiber laser sensor of this type is disclosed for example in G. A. Ball et al., “Polarimetric heterodyning Bragg-grating fiber-laser sensor”, Optic Letters 18 (22), 1993, pp. 1976-1978. The sensor has a fiber laser with two Bragg gratings and a doped birefringent fiber segment that is arranged in between, acting as laser medium. The Bragg gratings are written directly into the fiber core of an optical fiber and form so-called fiber Bragg gratings. Pump light which is passed to the doped fiber segment by means of a lead fiber through one of the Bragg gratings excites two orthogonally polarized polarization eigenmodes in the fiber segment. In the emission light of the laser, the two polarization eigenmodes are brought to interference as a result of which a common beat frequency is obtained. Any external disturbance which changes the length of the laser cavity or the birefringence leads to a change in said beat frequency.
The obtained beat frequency and its changes can be measured by means of a frequency counter, so that it is possible to draw conclusions about the magnitude of the external influence, for example an external pressure or a temperature change. However, this fiber laser sensor cannot distinguish between individual external influences. In particular, temperature and pressure effects are accumulated in the sensor signal.
WO 99/44023 discloses a fiber laser pressure sensor in which two sensor fiber segments with a nonrotationally symmetrical structure are present in the laser cavity in addition to a fiber segment acting as laser medium. If an external pressure is exerted on one of these sensor fiber segments, then a pressure-proportional beat frequency is again induced between different polarization modes. In order to compensate for temperature effects, it is proposed to expose both sensor fiber segments to the temperature but only one sensor fiber segment to the external pressure. This fiber laser sensor can also be used purely for temperature measurement by determining a shift in the Bragg wavelength of the fiber Bragg grating and thus the wavelength of the emission light by means of an optical wavelength meter. The use of such an additional device increases the overall costs of the sensor. Moreover, the temperature difference between the two fiber Bragg gratings must not be greater than 10 K since otherwise the laser emission collapses on account of the different Bragg wavelengths.
WO 94/17366 describes a fiber-optic sensor with a plurality of fiber lasers connected in series. For separate measurement of the temperature, it is proposed in this case to use two fiber lasers with different wavelengths and to expose both to the same temperature and pressure changes. In this case, both fiber lasers experience the same pressure-induced, but different temperature-induced, wavelength changes. By subtracting the two signals, it is possible to determine the temperature change. Furthermore, this publication proposes configuring the two Bragg gratings of the individual fiber lasers differently, so that each fiber laser has a narrowband and a broadband Bragg grating. By using Bragg gratings of different widths, only the narrowband Bragg grating determines the Bragg wavelengths. This is intended to prevent disturbances in the cavity and thus so-called mode hopping, caused by different expansion of the two Bragg gratings.
Alan D. Kersey et al., “Fiber Grating Sensors”, Journal of Lightwave Technology, Vol. 15, No. 8, 1997, pp. 1442-1463, discusses various active and passive fiber grating sensors, the use of chirped Bragg gratings for pressure and temperature measurement being disclosed.